1. Field of the Invention
The invention relates to anti-lock braking systems (“ABS”) and, in particular, to the tone ring component of such systems. More particularly, the invention is directed to an ABS tone ring of the type integrated into a disc brake rotor, wherein a corrosion-resistant layer is applied to the tone ring by an economical and efficient process.
2. Background Art
In recent years, anti-lock braking systems, more commonly referred to as ABS, have become common safety equipment on virtually every type of vehicle ranging from cars to semi-trailers. In heavy-duty vehicles such as straight trucks and semi-trailers, U.S. government regulations require at least one axle of the tandem axles commonly found on such vehicles, to include an ABS sensor on each end of that axle. Some manufacturers of such heavy-duty vehicles go beyond government regulations and include the ABS sensors on every axle to improve ABS performance.
Each ABS sensor must be located adjacent to the inboard facing end of the wheel hub. The tone ring can be attached directly to the inboard end of the wheel hub, or alternatively, can be incorporated into the disc brake rotor, which in turn is attached to the inboard end of the wheel hub. More specifically, the tone ring is circular and is formed with notches, protrusions, teeth, or similar divisions, hereinafter generally referred to as teeth, and rotates with the wheel hub or disc brake rotor to which it is attached or into which it is integrated, respectively. As is well-known to those skilled in the art, the ABS sensors detect certain movements, such as distance, of the tone ring teeth, which in turn causes an electrical signal to be sent to the ABS electronic control unit enabling control of brake engagement. Thus, it is important that the ABS sensor be precisely positioned on the vehicle axle inboardly adjacent to and aligned with the tone ring teeth so that it can properly perform its sensing function.
In vehicles that utilize disc brakes, space is limited in the area of the axle ends where wheels and brakes are mounted. Such space constraints make integration of the ABS tone ring into the disc brake rotor desirable and in some cases necessary. A common method for integrating the tone ring into the rotor is to integrally cast or machine the features of the tone ring directly into the rotor structure.
However, one drawback of integrating the tone ring into the disc brake rotor is that the tone ring is subjected to significant thermal fluctuations due to the heat expelled from the rotor during normal braking operations. More specifically, a rotor integrated tone ring may be subjected to peak temperatures in excess of 1100 degrees Fahrenheit (°F.), and regularly is exposed to temperatures in the 300-500° F. range. In addition, all tone rings, whether integrated into a disc brake rotor or mounted directly on a wheel hub, are exposed to the environment beneath the vehicle. More specifically, the conditions beneath a vehicle during its operation can be very harsh, such that a tone ring is subjected to numerous corrosive and oxidizing substances including, for example, magnesium chloride, which is one compound used to melt ice on roads during winter months. The combination of thermal fluctuations and harsh environmental conditions has been known to cause formation of oxidation and corrosion byproduct layers on the teeth of integrally formed disc brake rotor tone rings. The accumulation of such undesirable materials on the tone ring teeth can lead to degradation and eventual loss of the signal being sensed by the ABS sensor. This loss of sensitivity can, in turn, lead to an ABS failure, which is an undesirable condition requiring the vehicle to be serviced.
Conventional means of protecting the integrally formed disc brake rotor tone ring, such as plating or painting, are not practical. More particularly, the extremely high operating temperature of a disc brake rotor would damage protective paints, organic-based corrosion coatings or regular platings. Plating, coating or painting the entire rotor with a special material capable of withstanding such extreme conditions would interfere with the performance characteristics of the brake, and selective plating, coating or painting of the tone ring portion of the rotor would require expensive and difficult masking of the rotor to prevent degradation of braking performance characteristics. Plating also can cause hydrogen embrittlement, which can in turn possibly cause catastrophic rotor failure.
The present invention corrosion-resistant ABS tone ring and method for applying the corrosion-resistant layer to the tone ring, is intended to economically and efficiently prevent the formation of oxidation and corrosion byproduct layers on the teeth of the integrally formed disc brake rotor tone ring. This result is accomplished through local application of a thermally-sprayed or flame-sprayed layer of corrosion-resistant material to the teeth of the integrally formed disc brake rotor tone ring. Such local application is possible because thermal spraying uses a hypersonic plasma stream that can be directed at close range with a nozzle that keeps the coating from affecting surrounding areas. Thermal spraying thus allows economical but efficient localized depositing of the corrosion-resistant material on the critical tone ring surfaces without affecting the performance characteristics of the brake rotor surfaces.